use a proper tensor library

[Kwasniok]

• PRO:
• • library benefits
• CON:
• • less flexibility (?)
• candidates:
• • ~https://github.com/C-SWARM/ttl~ (no customizable dimension per rank)
• • ~https://itensor.org~ (primarily for Julia and research?)
• • ~https://gitlab.com/wlandry/ftensor~ (too slow)
• • https://github.com/Kwasniok/cpp-generic-tensor-template-library
• constraints:
• • constexpr / static typed
• • basic operations +,-,*
• • concise notation
• • scalar type is template parameter

[Kwasniok]

use https://github.com/Kwasniok/cpp-generic-tensor-template-library